The present invention relates to interconnect metal structures for semiconductor devices with three or more terminals that operate at relatively high power. Examples include metal semiconductor field effect transistors (MESFETs) formed in silicon carbide (SiC), high electron mobility transistors (HEMTs) formed in the Group III nitride material system and other such devices that operate at a nominal power dissipation of one watt per millimeter (1 W/mm) of gate periphery (or greater) or that experience thermal excursions of more than 150° C. under nominal operating conditions.
As generally well recognized in the semiconductor art, the performance characteristics and capabilities (or limitations) of a semiconductor device are based upon the characteristics of the semiconductor material. Although silicon and gallium arsenide (GaAs) are used for a wide range of semiconductor electronic devices, certain of their performance capabilities will be limited by their bandgap (e.g., 1.12 electron volts (eV) at 300 K for Si and 1.42 eV for GaAs) and by their physical properties (e.g., melting point). Accordingly, for higher power devices, wide bandgap materials such as silicon carbide, the Group III nitrides, and diamond are becoming preferred.
From an electronic standpoint, silicon carbide has a number of theoretical and practical advantages that make its use desirable in microelectronic devices. Silicon carbide has a wide bandgap (3.0 eV for alpha-SiC at 300 K), a high critical breakdown field (approximately 2 mega-volts per centimeter), and a high thermal conductivity (about five watts per centimeter-Kelvin). Silicon carbide is also physically very hard. Silicon carbide has a high electron drift velocity, excellent thermal stability, and excellent radiation resistance or “hardness.” These advantages have been recognized and described thoroughly in the patent and non-patent literature
The Group III nitride material system offers similar advantages, including wide bandgaps (e.g., 3.36 eV at 300 K for gallium nitride and 6.2 eV for AlN). Additionally, the Group III nitrides form several binary, ternary, and tertiary compounds with bandgaps between 3.4 and 6.2 eV based upon the specific atomic fractions. As a result, they provide the capability to form heterojunctions and related structures between Group III nitride materials. The GaN/AlGaN heterostructure is particularly useful for high electron mobility transistors (HEMTs).
Because such devices are most typically used in combination and in circuits, they are typically connected to other devices using some form of conductive pathways (“interconnects”). These interconnects (often formed of metals) within, between, and among semiconductor devices must be able to withstand the operating parameters—most typically current, power and heat (temperature)—desired or needed from such devices.
As one example among many that are possible, wide bandgap devices such as metal semiconductor field effect transistors and high electron mobility transistors are useful as MMIC (microwave integrated circuits) components that can produce increased power output at traditional radar frequencies (e.g., 16.7 Gigahertz for Ku-band radar) including long pulse radar systems. In such systems, however, metal interconnect systems have been observed to begin to fail as the power density increases. The problems also arise relatively quickly, particularly compared to the otherwise long life advantages of semiconductor electronics. For example, such MESFETs and HEMTs operating at between about 8 and 10 watts per millimeter (gate periphery) have been observed to fail as early as 10 million cycles. Because the devices cycle at about one millisecond per cycle, they will fail in a few hours.
In other applications such devices never need to operate at such power levels. Nevertheless, the failure of high power devices in a relatively short time frame indicates that the same problem will eventually occur in lower power devices and in a similarly unacceptably short period of time.
One aspect of the problem arises from the use of several metals to form an interconnect. Conventional systems will use, for example, a layer of a diffusion barrier metal such as molybdenum adjacent to the semiconductor. A layer of a more electrically conductive material such as gold, silver or aluminum is then layered onto the diffusion barrier metal. These conductive metals can, however, migrate between and among layers of adjacent material. This in turn causes problems such as undesired metallurgical reactions, voids, uneven interfaces and corrosion. As the name implies, the diffusion barrier metal prevents the high electrical conductivity metal from reacting with the semiconductor in an undesired fashion.
Additionally, sometimes the molybdenum layer or an alternative metal such as titanium will be included as an adhesion layer to help maintain the ohmic contact or interconnect in or to the device.
Because high power devices generate relatively high temperatures and high thermal cycles (for example, ranging over 150° C.) the thermal effects on these metals (and any other materials) must be taken into consideration. As a result, and because wider bandgap materials can operate at higher power than lower bandgap materials (for devices of otherwise similar size and structure), the thermal stresses on interconnect metals are greater in wide bandgap material devices than they are in silicon-based or gallium arsenide-based devices.
Expansion is, of course, one such thermal effect. The extent to which the material will expand depends upon the applied temperature and the coefficient of thermal expansion. As well understood in the art, the coefficient of linear thermal expansion is the ratio of the change in length per degree K to the length at 273K. When considering expansion in three dimensions, the coefficient of volume expansion is typically about three times the linear coefficient. Furthermore, the value of the coefficient is temperature dependent.
In general, semiconductor materials have relatively low coefficients of thermal expansion. Metals have higher coefficient of thermal expansion than semiconductors. Within metals, higher electrical conductivity metals tend to have significantly higher coefficient of thermal expansion than do materials of lower electrical conductivity. Diffusion barrier properties tend to be associated with lower coefficients of thermal expansion.
In order to produce ohmic character, adhesion, a diffusion barrier, and conductivity, a metal interconnect system on a semiconductor will often include a layer for each purpose; e.g., one layer for the ohmic contact, a second layer of a different material for adhesion, a third layer of yet another material to act as the diffusion barrier, and the fourth layer of another material to provide high conductivity. As a result, the interconnect will generally consist of three or four different materials with a relatively wide range of coefficients of thermal expansion. Thus, as a high-power wide bandgap device cycles over a given temperature range, the thermal expansion stress tends to cause the layers to delaminate from one another. In particular, the thermal effects create a shear stress (sideways) between layers as well as a principal (Z-direction) stress across the layers. In turn, the stress and other potential factors can lead to resistance increases, film delamination, passivation cracking, and catastrophic device failure.
Because improved devices are reaching power levels never previously seen in devices of equivalent size and structure made from conventional materials, the resulting delamination problems have not been observed. Thus, to some extent, these new problems are a result of success in the design and fabrication of higher power devices in wide bandgap materials.
Of course, the theoretical power capabilities of a device become insignificant if in actual use the associated materials such as metal interconnects tend to fail at a relatively early stage.
Accordingly, a need exists for metal interconnect systems that can withstand the thermal stresses generated by these higher performance devices.